1. Field of the Invention
The present invention relates to industrial fabrics, and particularly, but not exclusively, to papermachine fabrics. A preferred use of the fabrics of the present invention is as press felts for use In the press section of a papermaking machine.
2. Description of the Related Art
Paper is conventionally manufactured by conveying a paper furnish, usually consisting of an initial slurry of cellulosic fibres, on a forming fabric or between two forming fabrics in a forming section, the nascent sheet then being passed through a pressing section and ultimately through a drying section of a papermaking machine. In the case of standard tissue paper machines, the paper web is transferred from the press fabric to a Yankee dryer cylinder and then creped.
Paper machine clothing is essentially employed to carry the paper web through these various stages of the papermaking machine. In the forming section the fibrous furnish is wet-laid onto a moving forming wire and water is encouraged to drain from it by means of suction boxes and foils. The paper web is then transferred to a press fabric that conveys it through the pressing section, where it usually passes through a series of pressure nips formed by rotating cylindrical press rolls. Water is squeezed from the paper web and into the press fabric as the web and fabric pass through the nip together. Press fabrics generally comprise a batt of fibres needled to a base fabric. In the final stage, the paper web is transferred either to a Yankee dryer, in the case of tissue paper manufacture, or to a set of dryer cylinders upon which, aided by the clamping action of the dryer fabric, the majority of the remaining water is evaporated.
Most papermachine clothing is nowadays made from textile materials usually comprising polymeric yarns and or fibres. In an attempt to extend the lifespan and improve the performance of these fabrics GB 1,512,558 teaches the application of a resin coating to the fabric yarns, the resin coating being applied as a solution in organic solvent. However, the use of such solvents leads to unacceptable environmental problems. U.S. Pat. No. 4,439,481 relates to a press fabric to which one of a number of suitable synthetic polymeric resins is applied. Suitable polymeric resins are said to include polyolefins, such as polyethylene, ethylene copolymers, polypropylene, polyamides, fluorinated ethylene propylene, polyvinylchloride, polyvinylidene fluoride and acrylic polymers, B-stage thermosetting resins and liner epoxy resins. In the example, a fabric is immersed in a dip tank containing epoxy resin. The coating increases the stiffness of the press fabric and makes it more resistant to compression. This enhances the performance of the fabric in removing water from the paper web. However, the use of strong organic solvents is usually required in order to dissolve the epoxy resin prior to coating the fabric. This solvent must later be removed leading once again to environmental problems.
In U.S. Pat. No. 4,847,116 a press fabric is made by applying, to a base fabric, a homogeneous foam coating composed of resin particles, binder and solvent. The solvent is then evaporated by heat to fuse the resin particles to each other and to the base fabric. This method consumes considerable energy in providing the heat to evaporate the solvent.
In U.S. Pat. No. 4,571,359 a layer of particles of a synthetic polymeric resin is located on a base fabric. The particles are then sintered by heating so that they bond together and with the base fabric. Again this process involves a costly treating step. A similar method is described in U.S. Pat. No. 5,508,095 except in that a fabric is embedded within the sintered structure.
In U.S. Pat. No. 5,508,095 a layer of plastics powder material comprising soluble corpuscles is applied to a base fabric. By heat and pressure treatment a plastics layer is produced. The soluble corpuscles are leached out from this layer to provide through flow passages. Again this process involves a costly heating step.
In GB 2,200,867 additives are included in the needled batt layer of a papermakers felt so as to increase the contact area with the web. These additives are prone to wear and drastically reduce the belt porosity.